Previously, readout of laser gyroscopes has been accomplished by combining a small percentage of the counterrotating beams which is allowed to pass through a dielectric coated mirror in the ring laser path. Incidentally, the term "counterrotating beams" is used herein to describe laser beams propagating in opposite directions within a ring laser cavity. Also, the terms "light waves" or "light beams" as used herein, are not limited to radiant energy waves with wave lengths only in the visible range. As a general background reference and for a detailed discussion of this combining of laser beams, attention is directed to Laser Applications, edited by Monte Ross, Academic Press, Inc., New York, N.Y., 1971, pp. 134 to 200 relating to "The Laser Gyro," and especially pp. 139 to 141. Briefly, this combination has been accomplished using a prism which combines the beams at slightly differing angles and thereby creates a fringe pattern across a surface containing two photosensitive diodes, each diode being much smaller than an individual fringe.
Fringe spacing is determined by the equation: EQU D = .lambda./sin (.theta./2)
where D is the distance between adjacent maxima of the fringe pattern, .lambda. is a beam wavelength, and .theta. is the angle between the two beams. Typically, the fringe spacing is about 3 mm. and the two diodes are spaced about 90 degrees apart, which would be 25% of 3 mm. or about 3/4 mm. When the ring laser is rotated about its input axis, the counterrotating beam frequencies change slightly, one increasing and the other decreasing. The difference in frequencies results in a beat frequency which is manifest as the rate of movement of the fringe pattern across the photosensitive diodes. The diode outputs are fed into a logic circuit which determines rate and direction of angular motion.
Laser beam intensity has been controlled by varying the cavity length. This is accomplished by oscillating one of the mirrors in the laser path with a piezoelectric element and an A-C oscillator. With the two photosensitive detectors located behind one of the mirrors and at 90 degrees to each other, as noted above, each detector principally monitors beam intensity of one of the counterrotating beams. The output signals generated in the detectors are summed and fed into a circuit which adjusts the nominal cavity length to achieve maximum beam intensity. For a thorough discussion of this type of circuit, attention is directed to NASA report No. CR-132261, "Design and Development of the AA1300Ad02 Laser Gyro," by T. J. Podgorski and D. M. Thymian, 1973, pp. 10 and 11. The chief difficulty in summing the two signals is in matching the gains of the two photo-detectors employed. To make sure that the combined signal is truly made up of the correct proportions of the two photo-detector inputs, the gain of the two detectors must be matched with external electronics.